An axial turbine used for power generation includes, for example: a plurality of stator cascades fixed to a chamber; and a plurality of rotor cascades fixed to a rotor shaft. The stator cascade includes a plurality of turbine stator blades. The rotor cascade includes a plurality of turbine rotor blades.
Some turbine rotor blades include a T-shaped blade root section. The blade root section is fit in the blade groove formed on the rotor shaft, and thus the turbine rotor blade is fixed to the rotor shaft. The blade groove also has a T lateral cross-sectional shape corresponding to the shape of the blade root section. While the turbine is operating, centrifugal force acts on the turbine rotor blade. As a result, a contact surface of the blade root section, facing outward in a radial direction of the rotor shaft, contacts a bearing surface of the rotor shaft facing inward in the radial direction of the rotor shaft.
Japanese Patent Application Publication No. H7-63004 discloses the turbine rotor blade of this type. The turbine rotor blade has a step formed at a neck portion corresponding to a longitudinal bar of the T shape of the blade root section. The step is separated from a wall surface of the blade groove in a state where a rotor disk, forming a part of the rotor shaft, is stationary. The step is configured to contact the wall surface of the blade groove when the amplitude of the vibration of the turbine rotor blade increases while the turbine is operating. In this configuration, the frequency of the vibration of the blade can be changed by changing a boundary condition of the vibration of the turbine rotor blade. As a result, resonation with a certain exciting frequency can be prevented, whereby the reliability of the turbine rotor blade can be largely improved.
In the turbine rotor blade disclosed in Japanese Patent Application Publication No. H7-63004, the neck portion of the blade root section has one end extending outward, in the radial direction of the rotor shaft, from the outer circumferential surface of the rotor shaft. This portion has a length, in the axial direction of the rotor shaft, larger than a width of the neck portion in the blade groove, and serves as a platform section supporting the blade profile section.